docs: def joint_positions(angles, lengths, unit='rad'):
      """ Link object as defined by the standard DH representation.
  
      :param list angles: angles of each joint
  
      :param list lengths: length of each segment
  
      :returns: x positions of each joint, y positions of each joints, except the first one wich is fixed at (0, 0)
  
      .. warning:: angles and lengths should be the same size.
      """
      if len(angles) != len(lengths):
          raise ValueError('angles and lengths must be the same size!')
  
      if unit == 'rad':
          a = np.array(angles)
      elif unit == 'std':
          a = np.pi * np.array(angles)
      else:
          raise NotImplementedError
       
      a = np.cumsum(a)
      return np.cumsum(np.cos(a)*lengths), np.cumsum(np.sin(a)*lengths)